This invention relates to a process for improving the physical properties such as phosphor flow characteristics and x-ray packing properties of the niobium activated yttrium tantalate x-ray phosphor which in turn improves the image quality of the radiographs when used in intensifying screens. The improvements are obtained by coating the phosphor with a fine size silicon dioxide. This coated phosphoor exhibits a uniform packing of the phosphor particles without voids when used for measurement in measuring cells or in intensifying screens.
X-ray phosphors are used in x-ray intensifying screens which are used along with photographic film to enhance the photographic image formed on the film at the same time reducing the x-ray dose on the object during medical radiographic procedures. Phosphor materials used in these intensifying screens are to be colorless single phase materials with a polyhedral shape of well-defined crystal morphology. Also, the phosphors have to be good x-radiation absorbers and emit the light in the spectral region to which the film is sensitive. Generally, it is required that the phosphor particle size be about 4-11 micrometers in order to form a thin layer when drawn in the form of screens using certain binder solutions as media. In order to have high quality radiographic image quality, these screens have to be void free with smooth uniform layer of phosphor particle-binder solution drawn on the substrate. The substrate is usually a mylar sheet. In order to have smooth uniform packing layers of the phosphors, these materials are coated by certain coating agents and also some dispersant agents in order to create conditions that all the light emitted from the phosphor realized with minumum loss due to scattering, self absoptions, etc. It is also required that the phosphor particles be uniformly and closely packed on the screen with minimum voids which increases the image quality of the radiographs.
U.S. Pat. No. 4,225,653 describes the preparation and the crystal structure of M' phase rare earth tantalate materials and their luminescent properties. It also demonstrates that the M' YTaO.sub.4 phase is an efficient host for x-ray phosphors when activated with niobium and some rare earth ions compared to M phase YTaO.sub.4 (furgusonite). However, preparation and post firing processes are critical to obtain a good quality phosphor that gives enhanced phosphor brightness and flow characteristics that in turn result in improved radiographic quality when used in medical intensifying screens.